This invention, in one preferred embodiment, relates to an improved method of and apparatus for depositing a thin, highly electrically conductive, highly light transmissive, low melting point metal oxide film onto the surface of an amorphous semiconductor material to manufacture a photovoltaic device. Other preferred embodiments, disclosed herein, deal with the deposition of low melting point metal oxide films onto the surface of metal, glass and/or plastic substrates.
Recently, considerable efforts have been made to develop systems for depositing amorphous semiconductor alloys, each of which can encompass relatively large areas, and which can be doped to form p-type and n-type materials for the production of p-i-n and other type devices which are, in operation in photovoltaic and other applications, substantially equivalent to their crystalline counterparts.
It is now possible to prepare amorphous silicon alloys by glow discharge techniques that have (1) acceptable concentrations of localized states in the energy gaps thereof, and (2) provide high quality electronic properties. This technique is fully described in U.S. Pat. No. 4,226,898, Amorphous Semiconductors Equivalent To Crystalline Semiconductors, Stanford R. Ovshinsky and Arun Madan which issued Oct. 7, 1980 and by vapor deposition as fully described in U.S. Pat. No. 4,217,374, Stanford R. Ovshinsky and Masatsugu Izu, which issued on Aug. 12, 1980, under the same title. As disclosed in these patents, fluorine introduced into the amorphous silicon semiconductor operates to substantially reduce the density of the localized defect states therein and facilitates the addition of other alloying materials, such as germanium.
The concept of utilizing multiple cells, to enhance photovoltaic device efficiency, was discussed at least as early as 1955 by E. D. Jackson, U.S. Pat. No. 2,949,498 issued Aug. 16, 1960. The multiple cell structures therein discussed utilized p-n junction crystalline semiconductor devices. Essentially the concept is directed to utilizing different band gap devices to more efficiently collect various portions of the solar spectrum and to increase open circuit voltage (Voc). The tandem cell device has two or more cells with the light directed serially through each cell, with a large band gap material followed by one or more smaller band gap materials to absorb the light passed through the preceeding cell or layer.
It is of great commercial importance to be able to mass produce photovoltaic devices. Unlike crystalline silicon which is limited to batch processing for the manufacture of solar cells, amorphous silicon alloys can now be deposited in multipe layers over large area substrates to form solar cells in a high volume, continuous processing system. Continuous processing systems of this kind are disclosed, for example, in pending patent applications: Ser. No. 151,301, filed May 19, 1980 for A Method Of Making P-Doped Silicon Films and Devices Made Therefrom; Ser. No. 244,386, filed Mar. 16, 1981 for Continuous Systems For Depositing Amorphous Semiconductor Material: Ser. No. 240,493, filed Mar. 16, 1981 for Continuous Amrophous Solar Cell Production System; Ser. No. 306,146, filed Sept. 28, 1981 for Multiple Chamber Deposition And Isolation System and Method; and Ser. No. 359,825, filed Mar. 19, 1982 for Method and Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells. As disclosed in these applications, a substrate formed from stainless steel, for example, may be continuously advanced through a succession of deposition chambers, wherein each chamber is dedictated to the deposition of a specific material.
It is now also possible to produce amorphous semiconductor devices by a layering or clustering technique in which at least one density of states reducing element and band gap adjusting element is introduced without deletoriously affecting the electrical properties of the alloys. Such processes are disclosed in copending U.S. patent application Ser. No. 422,155 filed Sept. 23, 1982, entitled "Compositionally Varied Materials and Method For Synthesizing the Materials", Stanford R. Ovshinsky; and U.S. patent application Ser. No. 442,895, filed Nov. 19, 1982, entitled "Optimized Doped And Band Gap Adjusted Photoresponsive Amorphous Alloys And Devices", Stanford R. Ovshinsky and Masatsugu Izu.
Following the deposition of the semiconductor alloy layers, a further deposition process may be performed either in a separate environment or as a part of a continuous process. In this step, a thin, transparent or semitransparent film of electrically-conductive, light transmissive material such as TCO (transparent conductive oxide) of, for example, an alloy of indium, tin, and oxygen (ITO) is added. It is the process of and apparatus for depositing such a thin conductive, transmissive film atop a body of semiconductor material to which the present invention is primarily directed.
The deposition of such thin, electrically-conductive films which are also highly light transmissive in the visible range, has a variety of other important optical and electrical applications. These films may be used, inter alia, as: antistatic coatings; electrodes in photoconductor storage devices; liquid crystal and electrochromic displays; photothermal absorption devices; and active and passive layers in photovoltaic devices.
At the present time, thin transparent, conductive metal oxide coatings, used for the purposes outlined hereinabove, commonly comprise: tin oxide materials such as SnO.sub.2 and SnO.sub.2 doped with antimony or other elements; In.sub.2 O.sub.3 and In.sub.2 O.sub.3 doped with tin or other elements; or cadmium oxygen materials such as CdO and cadmium oxygen doped with tin. Note however, such materials as IN.sub.2 O.sub.3, SnO.sub.2 and ZnO are regarded as electrical insulators unless combined with a dopant, and/or formed in a manner which develops a large number of oxygen vacancies. While tin is commonly used to dope indium oxide, other metals, such as cadmium, bismuth and molybdenum may also be employed. Similarly, while antimony is commonly used to dope tin dioxide, metals such as cadmium, molybdenum and indium may also be employed.
The above materials, with tin as a dopant for the indium oxide and antimony as a dopant for the tin dioxide and having indices of refraction which minimize reflection, are particularly well suited for use as thin, conductive, transmissive films on semiconductor materials. This is true when they deposited in about 550 angstrom thicknesses which optimizes their "optical thickness".
Many fabrication processes have been employed to produce the high quality thin, transparent, conductive coatings discussed supra. A first production technique is a variant of a sputtering process in which d.c. or r.f. signals bombard metal-oxide targets and, thereby, eject the metal-oxide material of the targets for deposition onto a substrate. In a variation thereof, d.c. or r.f. sputtering signals are used with metal targets. This is accompanied by a post-ejection reaction in oxygen to create the material for deposition, the reaction occuring in a plasma generated by the sputtering signal. However, the foregoing processes involve high electrical and kinetic energies, relatively slow rates of deposition, and require post-deposition heating.
Another class of prior art fabrication processes involve the evaporation of a metal into the vapor zone of a vacuumized chamber for reaction of the metal with oxygen, the reaction being activated and enhanced by a plasma. This process is (1) disclosed by U.S. Pat. No. 4,336,277, filed Sept. 29, 1980; (2) described by P. Nath and R. F. Bunshah in a publication entitled "Preparation of In.sub.2 O.sub.3 and Tin-Doped In.sub.2 O.sub.3 Films By A Novel Activated Reactive Evaporation Technique", published in THIN SOLID FILMS, Vol 69 (1980); and (3) taught by P. Nath et al in a paper entitled "Electrical And Optical Properties Of In.sub.2 O.sub.3 : Sn Films Prepared By Activated Reactive Evaportion, published in THIN SOLID FILMS, Vol. 72 (1980). As taught by the foregoing, resistive heating is employed to evaporate the metal and an electron beam with a thermionic emitter is employed to generate the plasma. Pressure, in the range of 10.sup.-4 torr, is required to provide a sufficient number of metal and gas atoms colliding with the electrons to accomplish the metal-oxygen reaction. Further, it is essential that (1) an inert gas such as argon be introduced within the plasma zone to aid in the ionization of the oxygen atoms, and (2) that a magnetic field be employed to move the electrons through the zone in a helical path for increasing the length of time the electrons remain in the zone, thereby increasing the number of possible electron collisions with oxygen and metal atoms.
In summary of the prior art fabrication processes discussed hereinabove: (1) while the deposition of ultra-thin noble metals by evaporation or sputtering processes can provide a film exhibiting excellent electrical properties, the film lacks the requisite light transmissivity in the visible range; (2) the deposition of thin films by said fabrication processes may additionally require post-deposition oxidation by heating in oxygen; (3) the deposition of thin films by processes such as chemical vapor deposition, (deposition from a heated chemical vapor which typically includes a metal halide and H.sub.2 O vapor) requires elevated temperatures and includes the inherent, corrosive effects of halides, high stresses in the deposited film and impurities from the deposition atmosphere; and (4) the deposition of thin films comprising organometallics is costly.
The improved method of and apparatus for depositing thin, light transmissive, electrically-conductive coatings onto the surface of a substrate employs r.f. power to form the plasma from process gases introduced into a deposition chamber. The r.f. signal is particularly advantageous because the high frequency increases the number of metal atom-oxygen atom collisions necessary to generate an ionized plasma and the collision occurs at lower pressure than was possible with the methods and apparatus discussed supra.
The deposition process and apparatus described herein provides highly electrically conductive films which are highly transmissive in the visible range, formed of In.sub.2 O.sub.3 doped with tin, SnO.sub.2 doped with antimony, and of ZnO material. The results of tests preformed on these materials demonstrate the applicability of the process and apparatus to amorphous photovoltaic devices and crystalline photovoltaic devices which include a substrate, irrespective of whether that substrate is glass, metal or a synthetic plastic resin. The deposition rates which have been acheived are relatively high and the required substrate temperatures which have been achieved are relatively low.